This invention relates to a printing machine for corrugated board sheets, more particularly to a printing machine which is designed to facilitate the procedures of ink change in accordance with order changes in the printing of corrugated board sheets by using a low viscosity and highly quick-drying glycolic ink newly developed by the applicant, in which the residual ink of the previous order can effectively be recovered so as to minimize the amount of the ink to be wasted.
Generally, the conventional printing machines for corrugated board sheets can roughly be classified into two types: longitudinal feeding rotary press, as shown in FIG. 7 and flexographic press, as shown in FIG. 8, in which different structures are employed depending on the properties of the inks to be used. These two types of presses have merits and demerits which are substantially mutually contradistinctive, as will be described later.
To describe first the longitudinal feeding rotary press, "longitudinal feeding" means that the corrugated board sheet is fed to the press in the longitudinal direction which is orthogonal to the corrugation of the sheet. However, the rotary press is widely employed recently in a printer slotter in which the corrugated board sheet is fed transversely thereto to carry out printing and also slotting. In the printer slotter, a highly viscous glycolic ink is used, and the ink is transferred to printing dies mounted on a plate cylinder after the ink is milled homogeneously over a multiplicity of rubber rollers. For example, in a printer slotter consisting of a group of various kinds of rolls as shown in FIG. 7, a corrugated board sheet 16 is passed between a plate cylinder 12 having mounted thereon printing dies 10 and an impression cylinder 14 disposed to oppose to the plate cylinder 12 whereby to carry out printing in a desired pattern. Meanwhile, the ink in an ink fountain 20 is transferred through an ink fountain roll 22, an intermediate roll 24 and a series of transfer rolls 18 to the printing dies 10.
The printer slotter has merits and demerits of its own. These merits and demerits, which are basically attributable to the use of a highly viscous glycolic ink, will now be described below.
Merits:
(a) Since the ink is built up on the sheet, the printed letters or patterns are glossy.
(b) Since the ink is fed only to the necessary portions, consumption of the ink can be reduced (the ink is not circulated).
(c) When the ink is changed with another ink of a different color, the ink remaining on the rolls can be removed merely by scraping. Accordingly, the ink change operations can readily be performed in accordance with the order change, so that the printer slotter can cope with small lot printing in accordance with various types of orders. Unlike in the flexographic press to be described later, substantially no washing waste is discharged, requiring no expensive facility for treating such washing waste.
Demerits
(a) Most of the currently available glycolic inks are of quick-drying type, and studies are being made recently to further improve their quickness of drying. However, compared with the flexographic printing ink to be described later, the glycolic ink still takes a considerable time for drying after printing. Accordingly, the printer slotter cannot directly be connected to a die cutter or a folder gluer in the subsequent step
(b) Since the high-viscosity ink must be milled homogeneously, a number of rolls must be used for transferring the ink to make the mechanism complicated. If the rolls are worn, intricate procedures are required for adjusting the clearances between the rolls.
(c) Although the ink is transferred much to the sheet to allow printing at a high density just after the ink is supplied to the ink fountain, the ink to be transferred to the sheet is gradually reduced to allow printing at a low density as the ink is consumed. Thus, unevenness in the density of printing is liable to occur. Accordingly, an operator should constantly monitor the color density and perform intricate procedures of supplying the ink whenever the density of the ink drops.
To describe next the flexographic press, the flexographic press unlike the printer slotter uses a low-viscosity aqueous ink having a high fluidity, so that the ink dries very quickly, characteristically. For example, in the flexographic press also consisting of a group of rolls as schematically shown in FIG. 8, a corrugated board sheet 16 is passed between a plate cylinder 12 having mounted thereon printing dies 10 and an impression cylinder 14 disposed to oppose to the plate cylinder 12 whereby to carry out printing in a desired pattern. Further, an ink roll 26 and a squeezing roll 28 are rotatably disposed adjacent to the plate cylinder 12, and the ink supplied to the ink fountain defined by these two rolls 26,28 is transferred by the ink roll 26 to the printing dies 10 mounted on the plate cylinder 12. It should be noted here that the flexographic ink, unlike the one used in the printer slotter, is of highly quick-drying type, it must constantly be circulated so as to prevent drying and solidification thereof. For such purpose, a circulating mechanism is employed, as shown in FIG. 9, in which the ink contained in a tank 30 is supplied to the ink fountain defined by the rolls 26,28 via a pump 32 and a supply pipe 34, while the ink dwelling in the fountain is recovered from the ends of the rolls 26,28 to the tank 30 via a pair of dams 36 and recovery pipes 38.
Merits and demerits of the flexographic press will now be described below. These merits and demerits are also basically attributable to the properties of the ink to be used, which is of highly quick-drying and low viscosity.
Merit:
(a) The use of the quick-drying ink enables forwarding of the sheet to a die cutter or a folder gluer in the subsequent step immediately after printing.
(b) Since the ink is distributed evenly throughout the ink roll and the squeezing roll, no irregular color along the length of the rolls occurs in the printed matter. Accordingly, the operator need not constantly monitor the appearance of the printed matter.
(c) The mechanism of transferring the ink is very simple. Demerit:
(a) Since a system of constantly circulating the ink is employed, the rolls and the circulation system must be washed with a large amount of water whenever the ink is to be changed. Accordingly, it is impossible to fully recover the ink, so that a considerable amount of ink is inevitably wasted, and besides a facility for treating the washing waste must be provided in view of pollution prevention, leading to cost elevation.
(b) Since it takes some time for the ink change operation, the flexographic press is not suitable for small lot printing in accordance with various types of orders.
(c) The printed letters or patterns are less glossy than those printed in the printer slotter.
As described above, the printer slotter and the flexographic press have merits and demerits which are substantially mutually contradistinctive due to the properties of the inks to be employed respectively. The merits of the flexographic press are noted in the art, so that the printer slotter has been replaced predominantly by the flexographic press. Namely, it is because the industry recognized the merits of the flexographic press, in spite of the demerits of its own, over the printer slotter.
By the way, in recent years, the industry is urged to cope with small lot processing of corrugated board sheets in accordance with various types of orders, and this tendency is becoming conspicuous year by year. Thus, such small lot processing in the case of a printing machine means that the inks must be changed frequently within a limited time so as to cope with printing of corrugated board sheets in small lots in accordance with various types of orders. Under such circumstances, the flexographic printer is insufficiently provided for the ink change, as described in the paragraph of demerits thereof, so as to cope with the frequent order changes as the result of small lot printing. Besides, there remains a problem of treating the washing waste somehow.
It is true, as described above, that the printer slotter can suitably be employed for such small lot printing, since it requires a short ink change time, but it suffers problems in that it requires of the operator an experienced skill and intuition in order to maintain optimum appearance in the printed matters and that it takes some time for drying after printing and thus the printer slotter cannot directly be connected to the subsequent step. Accordingly, users are employing the printer slotter not always with satisfaction so as to cope with the small lot printing in accordance with various types of orders which is increasing in recent years, but merely based on the rather negative reason that it takes shorter ink change time than in the flexographic printer.
In other words, while there is an earnest potential demand of users for a printing machine which requires a reduced ink change time and thus can sufficiently cope with the small lot printing, which requires no operator for monitoring the appearance of the printed matters and which can directly be connected to the subsequent step after printing, such printing machine is not realized yet. The key to obtain a practical printing machine having the merits of both the printer slotter and flexographic press is to develop an ink which can satisfy the above-described requirements (like in the cases of the printer slotter and the flexographic press which are dependent on the properties of the inks to be employed respectively). In this regard, the applicant tackled with the improvement of the ink itself so as to meet the potential demand of the users under cooperation of an ink maker and was successful in developing a glycolic ink having quick-drying properties and a low viscosity which are almost comparable to those of the flexographic ink.
However, as the result of various tests made for the ink newly developed by the applicant, it was found that its characteristics cannot be exhibited to the maximum if they are used in the conventional printer slotter. On the other hand, although it is possible to use such ink in the flexographic press, the ink need not be circulated since the degree of low viscosity and that of quick drying are not as conspicuous as in the original flexographic ink. Accordingly, the provision of the ink circulating mechanism specific to the flexographic press becomes meaningless, and thus the press cannot cope with small lot printing since it requires washing of the mechanism, leading to waste of the ink. The inconveniences of the conventional two types of printing machines described above are very natural since they are of the structures which are designed to exhibit the characteristics of the high-viscosity glycolic ink and a low-viscosity aqueous ink to be employed therein respectively.